1. Field of the Invention
The present disclosure relates to a xerogel production method, and in particular to a silica xerogel production method.
2. Description of the Related Art
An aerogel means in a broad sense “a gel containing air as a dispersion medium”, which is produced by drying a suitable gel. Here, “aerogels” in a broad sense include aerogels in a narrow sense, namely, a xerogel and further a cryogel. Such gels in a narrow sense have different names depending on a difference in how the gels are dried. A gel produced by supercritical drying is called an aerogel, a gel produced by drying under a normal pressure is called a xerogel, and a gel produced by freeze-drying is called a cryogel. The reason for gels in a narrow sense having various names due to the difference in how the gels are dried is that a dry process of removing a solvent from a gel is the most important of processes for producing an aerogel.
A silica aerogel containing silica particles, which was first synthesized by S. S. Kistler in 1931 (Nature, 127, 741 (1931)), dose not include blowing gas having a low thermal conductivity, such as CO2 and cyclopentane, yet has a pore size equal to or smaller than a mean free path (68 nm) of molecules constituting air. Accordingly, silica aerogels are known to produce excellent heat insulating effects.
Silica aerogels are different from widely used heat insulating materials such as polyurethane (PU), expanded polystyrene (EPS), and a vacuum insulation panel (VIP). The heat insulating capability of silica aerogels shows almost no change over time. Furthermore, silica aerogels have heat resistance of 400° C. or more. Thus, silica aerogels are attracting attention as next-generation heat insulating material.
The heat insulating capability of PU and EPS foamed using gas having a low thermal conductivity falls due to the gas escaping over time. Furthermore, PU and EPS have poor heat resistance, which is a problem.
VIP has excellent heat insulating capability of several mW/mK. However, a small amount of air molecules mix over time from a portion heat-sealed when a core material is vacuum-enclosed. Accordingly, the degree of vacuum of VIP falls. Consequently, VIP has problems such as deterioration over time and heat resistance of about 100° C.
Silica aerogels are superior to existing heat insulating materials, in deterioration over time and heat resistance, and have an excellent thermal conductivity of about 15 mW/mK. However, silica aerogels have a network structure, like a string of beads, which includes several tens of nanometer-order silica particles in point contact, and thus has rather low mechanical strength. In view of this, in order to overcome the fragility, study has been made to improve strength of silica aerogels by combining, for instance, silica aerogels with fibers, a nonwoven fabric, and resin.
Inorganic nano porous body such as a silica aerogel is typically synthesized by a sol-gel method which is a solution phase reaction. The material of a silica aerogel is water glass (sodium silicate solution) and an alkoxysilane compound such as tetramethoxysilane. Hydrolysis is caused by a liquid medium such as water or alcohol, which is mixed with a catalyst as necessary. The gel material is polycondensed in a liquid medium, to form a wet gel (hydrogel, water glass containing water). After that, the wet gel undergoes a silylation reaction (optionally, following solvent displacement) and lastly, a liquid medium in the wet gel is evaporated and dried.
The drying techniques include supercritical drying and non-supercritical drying (normal pressure drying, freeze drying) mentioned above. Dynax Corporation (WO2007/010949) and Cabot Corporation (Japanese Patent No. 3854645), for instance, disclose synthesis of an inorganic nano porous body.
The process of synthesizing a wet gel from material is called aging. Aging is a process for forming a precise network structure of silica particles by causing a polycondensation reaction of silicic acid to proceed, which is generated by hydrolyzing water glass or an alkoxysilane compound. Aging is typically performed at a temperature ranging from 50° C. to 90° C. in an airtight container so that a liquid medium and a catalyst do not volatilize.